Nitrocellulose resulting from the nitration of cellulose contains residual acids which must be removed from the nitrocellulose. Nitrocellulose resulting from nitration of the cellulose has a high viscosity which must be reduced so that proper rheological properties are obtained. This is particularly necessary when employing nitrocellulose in propellant as a major component thereof in order to insure propellant stability and adequate mechanical properties. Illustrative prior art processes for stabilization of nitrocellulose containing residual acidic components are described below.
U.S. Pat. No. 1,818,733 describes a process for reducing the viscosity of nitrocellulose, increasing its stability and for purification. In the process described a mixture of nitrocellulose and water is pumped at a high velocity (about 2 feet per second) through a long coil at a pressure of about 80 psi. The temperature of the nitrocellulose and water is from about 130.degree. C. to 160.degree. C. but temperatures as high as 180.degree. C. are stated to be permissible. These conditions result in a reduction of the nitrogen content of the nitrocellulose and severely reduce the nitrocellulose viscosity to the point that it is unacceptable for normal propellant manufacture. The nitrocellulose is considered lacquer grade, suitable for use in plastics and paints. U.S. Pat. No. 1,911,201 discloses a modified operatus for use in conducting the process described in U.S. Pat. No. 1,818,733.
U.S. Pat. No. 2,366,880 discloses a process for the purification of nitrocellulose. In this process, preliminary steps of wringing, drowning, boiling and pulping of the nitrocellulose precede the inventive step of poaching. In the poaching step of the process the nitrocellulose is boiled first in an aqueous alkaline solution and then in water. The nitrocellulose is then separated from the liquor associated therewith after boiling continuously by continuously filtering the nitrocellulose forming a mat of nitrocellulose. The resulting mat is washed with water.
U.S. Pat. No. 2,776,965 relates to a process for the manufacture of nitrocellulose. In this invention, nitric acid esters of cellulose are produced by reacting cellulose with a nitrating mixture containing essentially nitric acid, magnesium nitrate and water, the ratio of magnesium nitrate to water being at least 1.2/1 and not more than about 2.2/1. After nitration the spent nitrating mixture is removed from the resulting nitric acid esters and stable nitric acid esters of cellulose are recovered directly.
U.S. Pat. No. 1,757,481 discloses an improvement in a method for reducing the viscosity of nitrocellulose. In the process disclosed nitrocellulose is boiled in water under pressure in an autoclave for at least 8 hours. In the process water is circulated into and out of the autoclave and from time to time an alkaline agent sufficient to maintain neutrality of the water is introduced into the circulating water. The water level in the autoclave is maintained at a level sufficient so that the nitrocellulose is immersed in the boiling solution. Undesired products are removed from the boiling solution by the introduction of steam under pressure below the water level and by the simultaneous withdrawal of any excess water at such level.
U.S. Pat. No. 2,077,455 discloses a process for reducing the quantity of residual acids in unpurified nitrated cellulose. In this process acid-wet nitrocellulose having a nitrogen content of 11.9% is first washed with 2 or 3 changes of water, the last wash being slightly alkaline, until the final water has an approximate pH value of 5 or higher. The nitrocellulose is then boiled with 10-15 times its weight of a 0.05% to 1% solution of an alkali or alkaline earth nitrite for from 3 to 4 hours. It is then washed substantially free from nitrite by means of 2 to 3 changes of fresh water.
U.S. Pat. No. 2,297,734 discloses a process for the stabilization of nitrocellulose which comprises water washing nitrocellulose to remove most of its free acid, stirring the nitrocellulose in a large excess of hot water for several hours, filtering the nitrocellulose, stirring the nitrocellulose into a cold 1-3% solution of sodium bicarbonate in water for from one hour to several days, filtering and washing the nitrocellulose with water until only a small amount of the sodium bicarbonate remains in the nitrated product, preferably an amount which produces a pH of 7.1 to 8.2 in the water. The resulting nitrocellulose is then stirred into a 0.1%-0.2% solution of dicyandiamide. The resulting nitrocellulose is said to be free from color and satisfactorily stabilized.